Conventional golf balls can be divided into two general classes: solid and wound. Solid golf balls include one-piece, two-piece (i.e., solid core and a cover), and multi-layer (i.e., solid core of one or more layers and/or a cover of one or more layers) golf balls. Wound golf balls typically include a solid, hollow, gas-filled, gel-filled or fluid-filled center, surrounded by a tensioned elastomeric material, and a cover. Solid balls have traditionally been considered more durable than wound balls, but many solid constructions lack the preferred “feel” provided by the wound construction.
By altering ball construction and composition, manufacturers can vary a wide range of playing characteristics, such as compression, velocity, feel, and spin, each of which can be optimized for various playing abilities. In particular, a variety of core and cover layer(s) constructions, such as multi-layer balls having double cover layers and/or dual core layers, have been investigated. These golf ball layers are typically constructed with a number of polymeric compositions and blends, including, but not limited to, polybutadiene rubber, polyurethanes, polyamides, and ethylene-based ionomers.
The core of solid golf balls is the “engine” of the ball, providing the velocity required for good distance. Too hard a core, however, can result in a golf ball that provides poor feel. Manufacturers are constantly experimenting with various core compositions and constructions in an effort to optimize both feel and distance. Most conventional solid cores comprise polybutadiene rubber (“BR”) or some modified form thereof, which provides the primary source of resiliency for the golf ball.
BR core compositions still have room for improvement in resilience, which is determined by coefficient of restitution (“COR”). Familiar to those skilled in the golf ball art, the COR along with angle of trajectory (i.e., launch angle) and clubhead speed determine the distance a golf ball will travel when hit by a golf club. One way to measure the COR is to propel a ball at a given speed against a hard massive surface and measure its incoming and outgoing velocity. The COR is the ratio of the outgoing velocity to the incoming velocity and is expressed as a decimal between zero and one. There is no United States Golf Association limit on the COR of a golf ball, but the initial velocity of the golf ball cannot exceed 250±5 feet/second. As a result, the industry goal for initial velocity is 255 feet/second, and the industry strives to maximize the COR without violating this limit.
In general, BR's of high molecular weight (high Mooney viscosity) have better resilience than BR's of low molecular weight (low Mooney viscosity). However, as the molecular weight increases, the milling and processing properties of the BR deteriorate. BR catalyzed with lanthanide series elements such as neodymium tends to be linear and narrow in polydispersity (close to 1.0). The narrow polydispersity allows high-molecular weight Nd—BR of to process readily, but the linearity causes problems in extrusion processes such as die swell and cold flow. BR catalyzed with cobalt and/or nickel, in comparison to Nd—BR, tends to be more branched and have wider polydispersity (distant from 1.0). While the branching characteristic facilitates processing, the wide polydispersity generally gives low resilience. Advantageously, blends of Co/Ni—BR and Nd—BR in core compositions enhance resilience in the resulting golf balls.
Attempts to improve golf ball COR by using various blends of BR in core compositions include, among others, U.S. Pat. Nos. 4,683,257; 4,931,376; 4.955,613; 4,984,803; 5,082,285; 6,139,447; 6,277,920; and 6,315,684. Although some of the core compositions described in these disclosures are satisfactory, a need remains for compositions with improved properties and processabilility to form golf balls.